> Then I did some calculations: How much effort to change mp3gain, so it
> does write the neccessary tags? It already provides an option for
> track or album gain, but obviously can only apply album or track gain,
> as only replaygain tags give you the possibility for both in the same
> file.

Not that much, I imagine. The one missing piece is the ability to
add/create the ID3V2 tag (filling it with ID3V1 information, if needed),
and there should be enough decent implementations for that around.